Separation of gas mixtures by means of membranes is well known. Industrial uses of membrane-based gas separation systems include the production of oxygen-enriched air, nitrogen production for blanketing, separation of carbon dioxide from methane, hydrogen recovery from various gas mixtures and removal of organic vapors from various gas mixtures. Design of a membrane system for any particular application can be a complex matter, where narrow specifications for the composition of gas streams entering and leaving the membrane system must be met and where operating parameters must be constrained within certain values.
In any design, an important consideration is the membrane feed gas flow rate. Upon this flow rate depend the membrane area required and the flow capacities of ancillary equipment, such as pumps, condensers, etc. Furthermore, fuctuations in feed flow can have a serious effect on the performance of the membrane system. If the feed flow fallsy substantially below the lower limit for which the membrane system was designed, there may be poor mixing in the feed gas, resulting in a boundary layer adjacent the membrane surface that is depleted in the faster permeating gas. In consequences, the degree of enrichment of that gas in the permeate decreases. If the feed flow rises substantially above the upper limit for which the membrane systemw as designed, the result is a diminished stage cut and poorer removal from the feed. Thus, membrane systems are relatively sensitive to fluctuations in feed gas flow and do not handle well streams characterized by surges of high gas flow or periods of very low gas flow.
Gas streams with nonconstant flow rates are produced by many processes. If a membrane system is to treat such a gas stream, one way to accomodate the fluctuations is to size the membrane system to treat the maximum possible flow. This often means that the membrane system is grossly oversized. Besides the inefficiencies and extra costs associated with underuse of equipment, this may result in the boundary layer problems described above. There are many situations where a membrane system could provide a technically and economically attractive solution to a gas separation problem, were it not for too widely fuctuating feed gas flow.